1. Field of the Invention
The present invention relates to a muffling apparatus having an exhaust emission purifying function of reducing NOx in the exhaust gas from, for example, a vehicle engine and the like to purify the exhaust gas using a liquid reducing agent and a reduction catalytic converter, and in particular, to a technology for improving the NOx reduction performance in a muffling apparatus having a structure for forcibly turning a flow direction of the exhaust gas back inside thereof and capable of exhibiting an exhaust emission purifying function in addition to the muffling function.
2. Description of the Related Art
As an exhaust emission purifying system for eliminating NOx contained in the exhaust gas of an engine, there has been proposed an exhaust emission purifying apparatus for injection-supplying a liquid reducing agent to a position located upstream of a reduction catalytic converter disposed in an engine exhaust system, so that NOx in the exhaust gas and the reducing agent is subjected to the catalytic-reaction to each other, thereby purifying NOx into a harmless component (refer to Japanese Unexamined Patent Publication No. 2000-27627).
In order to install such an exhaust emission purifying apparatus on a vehicle, such as a motor tractor, which is restricted in the length of an exhaust pipe thereof since the vehicle length thereof is short, there has been developed a muffling apparatus having an exhaust emission purifying function in addition to its inherent muffling function, due to incorporating therein an exhaust emission purifying apparatus.
As a muffling apparatus installed on a vehicle, such as a motor tractor, which is restricted in the length of an exhaust pipe thereof, there is proposed an apparatus having a structure in which a turning portion for forcibly turning the exhaust gas flow back to an exhaust gas flow passage therein is disposed, so that the entered exhaust gas is led to the exhaust gas flow passage on an upstream side of the turning portion, and thereafter, is forcibly turned back at the turning portion and is led to the exhaust gas flow passage on a downstream side to be emitted to the atmosphere. In the above-described exhaust emission purifying apparatus, in order to improve the NOx purifying efficiency by the reduction catalytic converter, the liquid reducing agent needs to be sufficiently diffused into and to be mixed with the exhaust gas until the liquid reducing agent reaches the reduction catalytic converter, and accordingly, a certain amount of distance is required to be furnished between an injection nozzle for the liquid reducing agent and the reduction catalytic converter. Therefore, in the case where the above-described exhaust emission purifying apparatus is incorporated in the above-described muffling apparatus having the structure in which the turning portion is disposed, it is desirable that the injection nozzle for the liquid reducing agent is disposed on the upstream side exhaust gas flow passage, and the reduction catalytic converter is disposed on the downstream side exhaust gas flow passage.
However, in the muffling apparatus of the structure in which the turning portion is disposed, since the structure thereof is such that the exhaust gas flow direction is forcibly changed at the turning portion, the flow of the exhaust gas flow must become unequal. Accordingly, the flow of the exhaust gas entering into the reduction catalytic converter is inconstant, which causes uneven distribution of the liquid reducing agent when it flows into the reduction catalytic converter and hence, there might be such a possibility that the NOx purification performance cannot be achieved satisfactorily.
Incidentally, it is possible to obtain a relatively even distribution of the liquid reducing agent in the exhaust gas, if the injection nozzle for the liquid reducing agent, which is arranged to protrude from a flow-passage-wall-face of the exhaust gas to the inside of the passage, is provided to extend in a manner such that a nozzle opening thereof is positioned on the center of the exhaust gas flow passage. Nevertheless, since the injection nozzle is supported by a cantilever supporting structure, there might be a possibility that the injection nozzle is damaged due to the vehicle body vibration or the like, and also, there is a possibility that the heat capacity of the injection nozzle necessarily becomes larger, so that the liquid reducing agent is attached to be accumulated on an inner wall of the injection nozzle due to the exhaust heat, resulting in clogging of the nozzle.